HYMENOPTERA— WASPS 167 



species of bee may visit flowers with exposed or partly concealed nectar more readily 

 than bee flowers (Osmia rufa Z. J ), while another may prefer bright instead of dark 

 colours (Heriades truncorum Z.). Neither the two sexes of the same species nor the 

 various species of the same genus, nor genera of the same family with proboscis of 

 equal length, are restricted in their visits to flowers, as might be theoretically 

 expected by the purely mechanical determinant afforded by the length of the proboscis. 

 Nest-building, early or late time of appearance, special preference of the larvae or 

 adults for pollen as food, and so forth ; all these factors influence the selection of flowers, 

 at least as much as this depends upon the structure and length of the proboscis in 

 the insect pollinator. The principle emphasized by Hermann MuUer of arranging bees 

 in an ascending series according to the length of the proboscis, from those in which 

 this organ is very short to those in which it is very long, is consequently one-sided, 

 and Loew has therefore undertaken a more comprehensive classification (cf 

 pp. 192-5). 



The other Hymenoptera 



play a much less important part as agents of cross-pollination than bees, which have 

 now been dealt with. The latter (except short-tongued species) are the most highly 

 organized visitors to flowers, and there are a large number of mutual adaptations 

 between their structure and that of flowers. Loew therefore terms the highly 

 specialized Apidae eutropous (i.e. well-adapted) insects, while he describes those 

 which are in a lower stage of specialization as hemitropous (i.e. half-adapted). 

 (Cf pp. 193-4.) Following Hermann MuUer (p. 149) it has already been emphasized 

 that the least specialized bees hardly stand on a higher morphological level as 

 regards adaptation to pollination than the fossorial wasps (Sphegidae). The latter, 

 however, are less concerned with flowers in that only the adults use these as a source 

 of food, for they do not provide their larvae with pollen and honey, but with spiders, 

 insects, or insect larvae, which have been paralysed or killed by stinging. The 

 structure of their mouth-parts, as already stated, agrees on the whole with that of 

 Prosopis and Sphecodes, but the mentum and maxilla are still shorter and less 

 narrow. A pollen-collecting apparatus does not occur, as is but natural considering 

 the way in which their young are fed. 



In their mode of visiting flowers, the Sphegidae never display the certainty, one 

 might almost say geniality, with which most bees are able to recognize the objects of 

 their predilection from considerable distances. This perhaps depends upon a lower 

 development of their olfactory organs. The visits of fossorial wasps are more 

 especially paid to the flowers of classes E, EC, C, S, and Hw. (Loew, ' Blumen- 

 besuch,' II, p. 98.) 



True wasps (Vespidae), considered as visitors to flowers, may be divided (Loew, 

 op. cit., II, p. 100) into two groups. The members of the first (including the social 

 genera Vespa and Polistes) nourish themselves only occasionally upon flower-food, 

 but also feed on the juices of plant-lice, sweet fruits and other edibles, raw beef, 

 sugar, and the soft parts of captured insects (flies, bees, Lepidoptera). The mem- 

 bers of the second group (including the solitary genera Eumenes, Discoelius, 

 Odynerus, and Pterocheilus), when adult, live upon flower-food only. There is 

 a corresponding difference in the formation of the ligula. It is only in the second 



